A variety of at least substantially resin-free, chromium-containing coating compositions for protecting ferrous substrates are known. Of special interest are those which contain particulate metal. Representative coating compositions of this type that were initially developed could be quite simplistic, such as compositions containing essentially chromic acid and particulate metal in an alcohol medium, as disclosed in the U.S. Pat. No. 3,687,738.
A later development of particular effectiveness for providing a corrosion-resistant coating on metal substrates was the more complex composition such as shown in U.S. Pat. No. 3,907,608. The composition comprised chromic acid, or equivalent, a particulate metal of mainly zinc or aluminum, wetter and a liquid medium comprising water plus high-boiling organic liquid. The composition had very desirable coating characteristics when including a viscosity modifier such as a water-soluble cellulose ether, as disclosed in U.S. Pat. No. 3,940,280.
The coating could be especially useful as an undercoating. Thus it has been taught to use such a more complex coating composition as an undercoating over ferrous surfaces. The coating is then provided with a silicate topcoating, as disclosed in U.S. Pat. No. 4,365,003. Another topcoating that could be utilized is a weldable primer, most notably a zinc-rich primer, which may be typically applied before electrical resistance welding of the substrate, as discussed in the hereinbefore mentioned U.S. Pat. No. 3,940,280.
It has been known that where coating compositions could contain the particulate metal as untreated aluminum flake, such flake can be unstable in water-based coating compositions. In such water-based coating compositions, standard aluminum flake will react with water in the composition to form hydrogen gas. One approach for avoiding this problem has been to coat the aluminum flake. One such coating is an acrylic coating formed by reacting mono-ethylenically unsaturated silane with acrylic monomers having amine hydroxyl or epoxy groups, as disclosed in U.S. Pat. No. 4,213,886. However, these products are specialty items tailored to provide a coating of good glamour appearance and have not found a wide acceptance.
Another approach to improving the coating composition was to consider the chromic acid constituent. As taught in U.S. Pat. No. 4,266,975 this constituency can be partially replaced by boric acid component. However, some chromic acid for the constituency is retained.
For coating compositions for providing corrosion-resistance to metal substrates, which compositions are of the particular type referred to as "wash primers", such have conventionally contained zinc chromate pigment. Attempts have been made with these primers to provide anti-corrosion primers free from chromium, thus reducing potential pollution problems. There has been proposed, as disclosed in U.S. Pat. No. 4,098,749, a coating composition containing a polyvinyl butyral resin, an organofunctional silane, a borate or polyphosphate compound and phosphoric acid. The composition may contain a metal powder as an optional ingredient and usually a phenolic resin. Such compositions, however, are not suitable as replacements for the above discussed complex compositions of pulverulent metal and chromium-providing substance, owing in part to their resin content.
As mentioned hereinabove, the corrosion-resistant coatings can be combinations of undercoatings and topcoatings. The topcoatings can be solvent-based weldable zinc-rich primers. For the topcoatings, such as these zinc-rich primers, it has been proposed, as disclosed in U.S. Pat. No. 4,476,260, to enhance primer corrosion-resistance by formulating a primer to contain zinc pigment, a thermoplastic or thermosetting resin, an organosilane, and optionally aluminum trihydrate with one or more dispersing agents. Such compositions, however, are not suitable as replacements for the complex undercoating compositions, and would be useful in the combination of coatings as the zinc-rich topcoating.
It would, therefore, be desirable to provide a coating composition which could have the wide acceptance of the complex undercoating compositions. It would further be desirable to provide such compositions which would avoid the pollution problems associated with the compositions which contain hexavalent chromium, as well as avoid compositions which are solvent-based.